Japanese Knotweed FAQs

By Paolo Martini on 1st May 2019 (updated: 6th October 2020)

What is Japanese knotweed?

Japanese knotweed (also known as Fallopia japonica or Polygonum cuspidatum) is a perennial herbaceous invasive plant that is a native species of Japan. Since being imported to botanic gardens in Britain during the Victorian era Japanese knotweed has spread throughout the UK and has been labelled as ‘controlled waste’ by the government. This plant grows aggressively during the summer and has the potential to grow through temporary structures (such as garden sheds or greenhouses) and can even rupture concrete.

Why is Japanese knotweed a problem?

Whilst the destructive potential of Japanese knotweed has been exaggerated by some journalists and removal companies, the plant can nevertheless prove to be a nuisance for homeowners, landlords and farmers alike. The plant dies back during the colder months of the year, but Japanese knotweed can grow at an alarming rate during the summer, leading to large patches of thick growth that can stifle other plants and quickly overrun an outdoor space.

Unfortunately, for those who find the plant on their land, getting rid of Japanese knotweed is not a simple process. Depending on how large the infestation is, the roots (rhizomes) of the Japanese knotweed can be submerged several metres underground. It might seem like a simple task to remove the foliage above ground but to properly eradicate the plant the entire rhizome system must be excavated. Due to its status as an invasive plant, there are also serious legal implications to having Japanese knotweed which can make resolving disputes with neighbours and treating the problem even more difficult.

Related: Japanese Knotweed Growing In Neighbour’s Garden: Legal Implications

What does Japanese knotweed look like?

Japanese knotweed enters several recognisable phases throughout the year, whilst treatment can take place at any time it’s typically easier to spot during the summer, as this is when the plant is most visible above ground. You’ll first spot small red-purple asparagus-like shoots growing in spring, these can grow up to 10cm a day during the summer and often grow in large patches.

Red-purple Japanese knotweed shoots

Knotweed shoots are a red-purple colour at first.

The leaves are the most distinguishable feature throughout the year. These are broad and shield-shaped with a distinctive alternating stem pattern, unfortunately there are other common plants that look like Japanese knotweeed. Both Lilac and Dogwood share similar shaped leaves. Look for small white creamy flowers and bamboo-like canes to be certain that you have a Japanese knotweed infestation, or hire a professional to survey your property.

Related: How To Identify Japanese Knotweed

Creamy white flowers of Japanese knotweed

Knotweed’s creamy white flowers are one of their distinguishing features.

What does Japanese knotweed look like in winter?

Knotweed can be difficult to spot during the winter without its recognisable leaves and flowers, which wilt and turn yellow when the weather gets colder. Its bamboo-like stems become hollow and brittle during the winter and change from a red/brown colour in autumn to a dark brown. These hollow stems soon collapse together and decompose, but the plant is still very much alive under the ground.

Dead canes of Japanese knotweed in winter

During the winter the hollow stems become brown and brittle.

How does Japanese knotweed grow?

Despite it flowering throughout the warmer months, this is not how knotweed reproduces in the UK. This invasive plant is such a problem because of its deeply embedded root system which is constituted of ‘rhizomes’. An entire infestation can manifest itself from the smallest of rhizome fragments. Construction sites, canals and railway lines are common sources of large infestations, however unknowing gardeners attempting to dig out the root can also exacerbate the problem.

Pulling out a Japanese knotweed rhizome

Rhizome root systems can be difficult to eradicate.

Related: How Does Japanese Knotweed Spread?

What time of year does Japanese knotweed grow?

Japanese knotweed does not grow throughout the year, like most plants it has a seasonal life cycle, developing through a number of phases in response to the changing environmental conditions. In order to correctly recognise Japanese knotweed, it’s useful to have a reference point to better understand these various stages of this plant’s life cycle.

Can Japanese knotweed just appear?

Japanese knotweed doesn’t appear from thin air. Like any other plant, its origins should always be able to be traced back to an original place. Discovering the source of a Japanese knotweed infestation is almost as important as making the initial positive identification. In order to determine where the plant has come from and when it first entered your land, you may need to consider whether the plant can be found anywhere in your local vicinity (in a neighbour’s garden or on adjacent publicly owned land). If there’s no sign of any knotweed near to your land, then it’s possible that the plant may have originated from a batch of contaminated soil that was dumped on the land, or accidentally transferred from footwear or a vehicle.

Is there Japanese knotweed in my area?

Japanese knotweed has been discovered all over the UK and is often grouped along canals, motorways and nearby areas that have been heavily redeveloped. Since the plant’s arrival in the UK in the 19th century, Japanese knotweed has been steadily disseminated throughout the country via unwitting gardeners and careless construction firms. Thanks to a public appeal made by the Environment Agency, we are now able to pinpoint which areas in the country have been affected by the plant and the results are not for the faint-hearted. Look at our Japanese knotweed map to find out if it has a foothold in your area yet.

How do you remove Japanese knotweed?

There are many methods of removing Japanese knotweed from your land. Which one you choose will depend on the size of the infestation that you’re dealing with, how quickly you want to clear your land and if there’s any other plant life or animals that are living in the area.

Herbicide treatment is ideal for large areas that are heavily infested and do not contain any living things that are in need of protection. Stem injection of concentrated Glyphosate herbicide is a targeted solution that is more time consuming, but also more effective. Digging out the infestation with machinery is costly, but can often be the only solution for large construction sites.

The use of a root barrier is intended to halt the spread of knotweed, rather than remove it entirely, whereas screening methods can be used to sift the soil and only remove the contaminated soil. Finally, incineration is one of the most effective methods of destroying the plant, but you must ensure that you are burning the entire infestation in order to prevent the problem returning.

How much does it cost to get rid of an infestation?

A professional spraying Japanese knotweed with glyphosate

Professional herbicide spray application is one treatment solution.

The cost of removing Japanese knotweed varies greatly depending on the size and severity of your infestation. In cases where you have not been properly informed of the knotweed, or where the plant has entered your land from a neighbouring property, you might not be liable to pay for the treatment at all.

Call us today to request a survey of your land, so that we can help determine the costs of removing the infestation and whether or not you could claim compensation to cover the treatment.

How long does it take to get rid of Japanese knotweed?

Treatment can take anywhere from 6 months to several years before proving to be effective and is heavily dependant on the size and mass of your infestation. We only work with Property Care Association (PCA) accredited removal experts who are able to give you 5 or 10 year certification in regards to your knotweed treatment.

Is it possible to get rid of it yourself?

We would strongly urge you not to attempt to treat your knotweed yourself for two reasons:

  • You’re likely to not carry out an effective treatment by yourself (and could possibly make the situation worse).
  • Also, without a certified Knotweed Management Plan, a mortgage company will not offer a mortgage on a property, regardless if the problem has been treated or not.

Although there are a number of DIY knotweed treatment solutions proffered online (bleach and vinegar being the most popular), there is no evidence that any of these are as effective as hiring a professional certified team. Glyphosate herbicide solution is available to purchase at retail stores, however commercial weed control products are not as powerful as the industrial strength solutions available to professionals and you are likely to only partially treat the problem.

If you want to get rid of your Japanese knotweed organically, then you can dig the roots out manually but once more there is no guarantee that you will complete the job. Property owners seeking to do so must also adhere to strict Japanese knotweed laws regarding the removal of this controlled substance, or risk facing serious punishments.

Related: A Guide To Getting Rid Of Japanese Knotweed

How do you properly dispose of Japanese knotweed?

Incorrectly disposing of Japanese knotweed or allowing any rhizomes from your waste to spread into the wild can lead to a fine of up to £5,000 or a prison sentence of up to 2 years. Regardless of if you hire a professional team to help you or not, you must use a registered waste carrier to transport your contaminated soil to a licensed landfill site. You should also call the landfill before you transport the waste, as it must have the correct environmental permit to deal with the knotweed.

Is Japanese knotweed dangerous?

Japanese knotweed can prove to be an expensive, time-consuming problem to deal with, however it is not dangerous to humans. Japanese knotweed is not poisonous, in fact, it’s edible and can be consumed easiest when it’s first shooting in spring.

Should I buy a house with Japanese knotweed?

Japanese knotweed growing in a suburban back garden

Properties with knotweed present lose 10% of their value.

If you buy a property with a Japanese knotweed infestation then you are inheriting years of treatment and the costs that go with it. An infestation of knotweed typically reduces a domestic property’s value by 10%. Some mortgage lenders have specific policies regarding this plant, so it’s worth enquiring with them before committing.

A knotweed infestation should be identified by the seller of your home on the TA6 form. If you have spotted knotweed on the property, but the seller has answered ‘Not Known’ on the survey form then you should bring this issue up with them straight away. Unfortunately, in some cases, property sellers lie about Japanese knotweed in order to push through a quick sale.

Find out about what happens if you’ve bought a property with knotweed in this case study.

How about selling a house with Japanese knotweed?

Conversely, if you are aware of a Japanese knotweed infestation on your property you must make potential buyers fully aware of this by noting it on the TA6 information form. Failing to do so could lead to a lawsuit that could cost you thousands of pounds. Similarly, if a property surveyor misses Japanese knotweed on your land and a buyer makes a claim against you, you may able to claim against the surveyor for professional negligence.

Read: Selling a Property with Japanese Knotweed

What do you do if you find Japanese knotweed in your garden?

It is not illegal to allow Japanese knotweed to grow on your land, so you are not required by law to notify anyone about an infestation. However, it is certainly in your interest to deal with the problem as soon as possible.

This plant is controlled by both the Anti-social Behaviour Crime and Policing Act 2014, the Environmental Protection Act 1990 and the Wildlife and Countryside Act 1981. Whilst harbouring the plant on your property is not illegal, allowing a knotweed infestation to spread into another property is and could lead to fines and lawsuits being brought against you. Leaving the plant to grow in size will only lead to your overall costs increasing with it.

Read our guidelines to find more about the laws controlling Japanese knotweed.

Who is responsible for clearing Japanese knotweed?

The responsibility for removing or clearing the Japanese knotweed rests on the individual whose land the plant originated on. For example, if knotweed in your neighbour’s garden has spread to your land then it would be their responsibility to pay for the treatment of the infestation. In order to claim for the costs of removal, you will need to prove that the infestation came from their land.

What is the limitation period for Japanese knotweed claims?

If you are considering making a claim for damages, it is important to seek advice as soon as possible. This is because legislation requires that you must commence your Japanese knotweed claim within certain time limits after discovering its presence. After this, it will almost always be too late.

Get in touch with us if you have any questions about how to deal with Japanese knotweed, we work on a No-Win No-Fee agreement and never take on a case that we don’t think we can win.

Paolo Martini

Paolo Martini is the lead solicitor for Knotweed Help and has over 30 years of experience in the field of Civil Litigation and is an expert on the legal issues faced by individuals dealing with Japanese knotweed on their land. Now considered one of the country's leading litigators in Japanese knotweed law he works alongside the country’s top barristers and experts. His in-depth legal experience and connections to the Japanese knotweed removal industry make him uniquely suited for handling your case.